The invention relates to a force limiter for a seat belt retractor.
Force limiters are typically used in a seat belt retractor comprising a frame, a belt reel rotatably mounted in the frame and a drum which may be coupled with the belt reel for joint rotation, and a plastically deformable metal band that may be wound on the drum with dissipation of energy, the metal band running through a plurality of deflection elements arranged on the frame to be deformed thereby. The mode of operation of such a force limiter is based on the fact that a limited rotation of the belt reel in the unreeling direction is permitted in a vehicle crash situation when the belt is loaded beyond a predetermined critical value, the metal band being drawn through the deflection elements and thereby being plastically deformed. The force limiting level is dictated, on the one hand, by the stiffness of the metal band and, on the other, by the geometry of the arrangement of the deflection elements. Prior art proposed defining the force limiting level selectively between at least two values for adapting to the stature data of the occupant. Selection may be done by movement of at least one of the deflection elements. Since this movement of the deflection element occurs against the metal band high positioning forces are needed.
The invention provides a force limiter for belt retractors in which making the selection from a higher to a lower force level requires less driving energy.
In accordance with a first embodiment of the invention this is achieved in a force limiter for a seat belt retractor comprising a frame, a belt reel rotatably mounted in the frame and a drum which may be coupled with the belt reel for joint rotation. The force limiter further comprises a plastically deformable metal band that may be wound on the drum with dissipation of energy. The metal band runs through a plurality of deflection elements arranged on the frame to be deformed thereby. At least one of the deflection elements is supported on a supporting structure fixed to the frame and is arranged on the frame so as to be movable transversely to the metal band. The deflection element is provided with a stepped contact surface area for a correspondingly stepped abutment surface area on the supporting structure, whereby, when the supporting structure engages a step of the contact surface area, a retraction movement of the deflection element relative to the metal band is permitted. The movement of the deflection element occurs initially only transversely to the direction of the supporting force, requiring only friction forces to be overcome, thus a low driving force being sufficient for the movement of the deflection element. As soon as the supporting structure then engages a step of the contact surface area of the deflection element, the deflection element is urged by the effect of the supporting force into its selected position corresponding to the lower force level until it is stopped by the abutment surface area of the supporting structure where it is safely locked in place.
In accordance with a second embodiment of the invention the above object is achieved in a force limiter in which at least one of the deflection elements is supported on a supporting structure which has an abutment surface area and is arranged on the frame so as to be movable substantially parallel to the abutment surface area, the deflection element comprising a stepped contact surface area for the correspondingly stepped abutment surface area, whereby, when the supporting structure engages a step of the contact surface area, a retraction movement of the deflection element relative to the metal band is permitted. In this embodiment of the invention the supporting structure is first moved transversely to the direction of the supporting force or, in other words, substantially parallel to the abutment surface area. Here again only friction forces need to be overcome so that a correspondingly low driving force is sufficient to move the supporting structure. When subsequently the stepped abutment surface area of the supporting structure engages a step of the contact surface area of the deflection element, the deflection element is able to give way under the effect of the supporting force and to move into its selected position corresponding to the lower force level until it is stopped by the abutment surface area of the supporting structure where it is safely locked in place.
The movement of the deflection element in the first embodiment or of the supporting structure in the second embodiment transversely to the direction of the supporting force is implemented preferably by an actuator. Such an actuator may be formed by a pyrotechnic piston/cylinder linear drive, by a solenoid or also by a preloaded spring.